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                    <h1 class="description center-align post-title">浅谈 Redis 键过期策略</h1>
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                <h1 id="浅谈-Redis-键过期策略"><a href="#浅谈-Redis-键过期策略" class="headerlink" title="浅谈 Redis 键过期策略"></a>浅谈 Redis 键过期策略</h1><p>在 Redis 中，我们都知道 Redis 的键值对都是存储在内存中的，要是我们一直往 Redis 中添加键值对，最后就会导致 OOM，所以我们需要定期的去删除一些键值对，来保证内存不会爆，此时我们就需要了解 Redis 的键过期策略了</p>
<p>在学会键过期策略之前，我们还是先来讲解如何使用键过期策略吧😄😄😄</p>
<h2 id="一、过期设置"><a href="#一、过期设置" class="headerlink" title="一、过期设置"></a>一、过期设置</h2><p>Redis 中设置过期时间主要通过以下四种方式：</p>
<ul>
<li>expire key seconds：设置 key 在 n 秒后过期；</li>
<li>pexpire key milliseconds：设置 key 在 n 毫秒后过期；</li>
<li>expireat key timestamp：设置 key 在某个时间戳（精确到秒）之后过期；</li>
<li>pexpireat key millisecondsTimestamp：设置 key 在某个时间戳（精确到毫秒）之后过期；</li>
</ul>
<p>下面我们依次来演示这几种过期设置吧</p>
<h3 id="expire：N-秒后过期"><a href="#expire：N-秒后过期" class="headerlink" title="expire：N 秒后过期"></a>expire：N 秒后过期</h3><pre class="line-numbers language-shell" data-language="shell"><code class="language-shell"><span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> <span class="token builtin class-name">set</span> key value
OK
<span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> expire key <span class="token number">100</span>
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> <span class="token number">1</span>
<span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> ttl key
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> <span class="token number">97</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>其中命令 ttl 的全称是 Time To Live，表示此键值在 n 秒后过期。例如，上面的结果 97 表示 key 在 97s 后过期。</p>
<h3 id="pexpire：N-毫秒后过期"><a href="#pexpire：N-毫秒后过期" class="headerlink" title="pexpire：N 毫秒后过期"></a>pexpire：N 毫秒后过期</h3><pre class="line-numbers language-shell" data-language="shell"><code class="language-shell"><span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> <span class="token builtin class-name">set</span> key2 value2
OK
<span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> pexpire key2 <span class="token number">100000</span>
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> <span class="token number">1</span>
<span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> pttl key2
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> <span class="token number">94524</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>其中 <code>pexpire key2 100000</code> 表示设置 key2 在 100000 毫秒（100 秒）后过期。</p>
<h3 id="expireat：过期时间戳精确到秒"><a href="#expireat：过期时间戳精确到秒" class="headerlink" title="expireat：过期时间戳精确到秒"></a>expireat：过期时间戳精确到秒</h3><pre class="line-numbers language-shell" data-language="shell"><code class="language-shell"><span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> <span class="token builtin class-name">set</span> key3 value3
OK
<span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> expireat key3 <span class="token number">1573472683</span>
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> <span class="token number">1</span>
<span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> ttl key3
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> <span class="token number">67</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>其中 <code>expireat key3 1573472683</code> 表示 key3 在时间戳 1573472683 后过期（精确到秒），使用 ttl 查询可以发现在 67s 后 key3 会过期。</p>
<blockquote>
<p>小贴士：在 Redis 可以使用 time 命令查询当前时间的时间戳（精确到秒），示例如下：</p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell"><span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> <span class="token function">time</span>
<span class="token number">1</span>. <span class="token string">"1573472563"</span>
<span class="token number">2</span>. <span class="token string">"248426"</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>
</blockquote>
<h3 id="pexpireat：过期时间戳精确到毫秒"><a href="#pexpireat：过期时间戳精确到毫秒" class="headerlink" title="pexpireat：过期时间戳精确到毫秒"></a>pexpireat：过期时间戳精确到毫秒</h3><pre class="line-numbers language-shell" data-language="shell"><code class="language-shell"><span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> <span class="token builtin class-name">set</span> key4 value4
OK
<span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> pexpireat key4 <span class="token number">1573472683000</span>
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> <span class="token number">1</span>
<span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> pttl key4
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> <span class="token number">3522</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>其中 <code>pexpireat key4 1573472683000</code> 表示 key4 在时间戳 1573472683000 后过期（精确到毫秒），使用 ttl 查询可以发现在 3522ms 后 key4 会过期。</p>
<h3 id="字符串中的过期操作"><a href="#字符串中的过期操作" class="headerlink" title="字符串中的过期操作"></a>字符串中的过期操作</h3><p>字符串中几个直接操作过期时间的方法，如下列表：</p>
<ul>
<li>set key value ex seconds：设置键值对的同时指定过期时间（精确到秒）；</li>
<li>set key value px milliseconds：设置键值对的同时指定过期时间（精确到毫秒）；</li>
<li>setex key seconds valule：设置键值对的同时指定过期时间（精确到秒）。</li>
</ul>
<p>实现示例如下:</p>
<p><strong>1. set key value ex seconds</strong></p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell"><span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> <span class="token builtin class-name">set</span> k <span class="token function">v</span> ex <span class="token number">100</span>
OK
<span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> ttl k
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> <span class="token number">97</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p><strong>2. set key value ex milliseconds</strong></p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell"><span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> <span class="token builtin class-name">set</span> k2 v2 px <span class="token number">100000</span>
OK
<span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> pttl k2
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> <span class="token number">92483</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p><strong>3. setex key seconds valule</strong></p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell"><span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> setex k3 <span class="token number">100</span> v3
OK
<span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> ttl k3
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> <span class="token number">91</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<h2 id="二、移除过期时间"><a href="#二、移除过期时间" class="headerlink" title="二、移除过期时间"></a>二、移除过期时间</h2><p>当我们设置键的过期时间之后，其实我们还可以移除键的过期时间，可以使用命令 <code>persist key</code> 可以移除键值的过期时间，如下代码所示：</p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell"><span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> ttl k3
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> <span class="token number">97</span>
<span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> persist k3
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> <span class="token number">1</span>
<span class="token number">127.0</span>.0.1:637<span class="token operator"><span class="token file-descriptor important">9</span>></span> ttl k3
<span class="token punctuation">(</span>integer<span class="token punctuation">)</span> -1<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>可以看出第一次使用 ttl 查询 k3 会在 97s 后过期，当使用了 persist 命令之后，在查询 k3 的存活时间发现结果是 -1，它表示 k3 永不过期。</p>
<p>其他过期操作方法：</p>
<ul>
<li>pexpire(String key, long milliseconds)：设置 n 毫秒后过期；</li>
<li>expireAt(String key, long unixTime)：设置某个时间戳后过期（精确到秒）；</li>
<li>pexpireAt(String key, long millisecondsTimestamp)：设置某个时间戳后过期（精确到毫秒）；</li>
</ul>
<h2 id="三、持久化中的过期键"><a href="#三、持久化中的过期键" class="headerlink" title="三、持久化中的过期键"></a>三、持久化中的过期键</h2><h3 id="RDB-中的过期键"><a href="#RDB-中的过期键" class="headerlink" title="RDB 中的过期键"></a>RDB 中的过期键</h3><p>DB 文件分为两个阶段，RDB 文件生成阶段和加载阶段。</p>
<p><strong>1. RDB 文件生成</strong></p>
<p>从内存状态持久化成 RDB（文件）的时候，会对 key 进行过期检查，过期的键不会被保存到新的 RDB 文件中，因此 Redis 中的过期键不会对生成新 RDB 文件产生任何影响。</p>
<p><strong>2. RDB 文件加载</strong></p>
<p>RDB 加载分为以下两种情况：</p>
<ul>
<li>如果 Redis 是主服务器运行模式的话，在载入 RDB 文件时，程序会对文件中保存的键进行检查，过期键不会被载入到数据库中。所以过期键不会对载入 RDB 文件的主服务器造成影响；</li>
<li>如果 Redis 是从服务器运行模式的话，在载入 RDB 文件时，不论键是否过期都会被载入到数据库中。但由于主从服务器在进行数据同步时，从服务器的数据会被清空。所以一般来说，过期键对载入 RDB 文件的从服务器也不会造成影响。</li>
</ul>
<h3 id="AOF-中的过期键"><a href="#AOF-中的过期键" class="headerlink" title="AOF 中的过期键"></a>AOF 中的过期键</h3><p><strong>1. AOF 文件写入</strong></p>
<p>当 Redis 以 AOF 模式持久化时，如果数据库某个过期键还没被删除，那么 AOF 文件会保留此过期键，当此过期键被删除后，Redis 会向 AOF 文件追加一条 DEL 命令来显式地删除该键值。</p>
<p><strong>2. AOF 重写</strong></p>
<p>执行 AOF 重写时，会对 Redis 中的键值对进行检查已过期的键不会被保存到重写后的 AOF 文件中，因此不会对 AOF 重写造成任何影响。</p>
<h2 id="四、主从库的过期键"><a href="#四、主从库的过期键" class="headerlink" title="四、主从库的过期键"></a>四、主从库的过期键</h2><p>当 Redis 运行在主从模式下时，从库不会进行过期扫描，从库对过期的处理是被动的。也就是即使从库中的 key 过期了，如果有客户端访问从库时，依然可以得到 key 对应的值，像未过期的键值对一样返回。</p>
<p>从库的过期键处理依靠主服务器控制，主库在 key 到期时，会在 AOF 文件里增加一条 del 指令，同步到所有的从库，从库通过执行这条 del 指令来删除过期的 key。</p>
<h2 id="五、过期策略原理分析"><a href="#五、过期策略原理分析" class="headerlink" title="五、过期策略原理分析"></a>五、过期策略原理分析</h2><p>讲完了过期键设置的基本使用，我们更加需要知道它的原理😂😂😂</p>
<h3 id="过期键执行流程"><a href="#过期键执行流程" class="headerlink" title="过期键执行流程"></a>过期键执行流程</h3><p>Redis 之所以能知道那些键值过期，是因为在 Redis 中维护了一个字典，存储了所有设置了过期时间的键值，我们称之为过期字典。</p>
<p>过期键判断流程如下图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327120724146.png" alt="image-20220327120724146" style="zoom:80%;" />

<p>过期键数据结构（字典）如下图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327120728757.png" alt="image-20220327120728757" style="zoom:80%;" />

<h2 id="六、过期策略"><a href="#六、过期策略" class="headerlink" title="六、过期策略"></a>六、过期策略</h2><p>Redis 会删除已过期的键值，以此来减少 Redis 的空间占用，但因为 Redis 本身是单线的，如果因为删除操作而影响主业务的执行就得不偿失了，为此 Redis 需要制定多个（过期）删除策略来保证糟糕的事情不会发生。</p>
<p>常见的过期策略有以下三种：</p>
<ul>
<li>定时删除</li>
<li>惰性删除</li>
<li>定期删除</li>
</ul>
<p>下面分别来看每种策略有何不同吧</p>
<h3 id="定时删除"><a href="#定时删除" class="headerlink" title="定时删除"></a>定时删除</h3><p>在设置键值过期时间时，创建一个定时事件，当过期时间到达时，由事件处理器自动执行键的删除操作。</p>
<ul>
<li><strong>优点：</strong>保证内存可以被尽快地释放。</li>
<li><strong>缺点：</strong>在 Redis 高负载的情况下或有大量过期键需要同时处理时，会造成 Redis 服务器卡顿，影响主业务执行。</li>
</ul>
<h3 id="惰性删除"><a href="#惰性删除" class="headerlink" title="惰性删除"></a>惰性删除</h3><p>不主动删除过期键，每次从数据库获取键值时判断是否过期，如果过期则删除键值，并返回 <code>null</code>。</p>
<ul>
<li><strong>优点：</strong>因为每次访问时，才会判断过期键，所以此策略只会使用很少的系统资源。</li>
<li><strong>缺点：</strong>系统占用空间删除不及时，导致空间利用率降低，造成了一定的空间浪费。</li>
</ul>
<h3 id="定期删除"><a href="#定期删除" class="headerlink" title="定期删除"></a>定期删除</h3><p>每隔一段时间检查一次数据库，随机删除一些过期键。</p>
<p>Redis 默认每秒进行 10 次过期扫描，此配置可通过 Redis 的配置文件 redis.conf 进行配置，配置键为 hz 它的默认值是 <code>hz 10</code>。</p>
<blockquote>
<p> 小贴士：Redis 每次扫描并不是遍历过期字典中的所有键，而是采用随机抽取判断并删除过期键的形式执行的。</p>
</blockquote>
<p><strong>定期删除流程</strong></p>
<ol>
<li>从过期字典中随机取出 20 个键；</li>
<li>删除这 20 个键中过期的键；</li>
<li>如果过期 key 的比例超过 25%，重复步骤 1。</li>
</ol>
<p>同时为了保证过期扫描不会出现循环过度，导致线程卡死现象，算法还增加了扫描时间的上限，默认不会超过 25ms。</p>
<p>定期删除执行流程，如下图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327120734718.png" alt="image-20220327120734718" style="zoom:80%;" />

<ul>
<li><strong>优点：</strong>通过限制删除操作的时长和频率，来减少删除操作对 Redis 主业务的影响，同时也能删除一部分过期的数据减少了过期键对空间的无效占用。</li>
<li><strong>缺点：</strong>内存清理方面没有定时删除效果好，同时没有惰性删除使用的系统资源少。</li>
</ul>
<blockquote>
<p>小贴士：Redis 使用的是惰性删除加定期删除的过期策略。</p>
</blockquote>
<h2 id="七、小结"><a href="#七、小结" class="headerlink" title="七、小结"></a>七、小结</h2><p>本文我们知道了 Redis 中的四种设置过期时间的方式：<code>expire</code>、<code>pexpire</code>、<code>expireat</code>、<code>pexpireat</code>，其中比较常用的是 <code>expire</code> 设置键值 n 秒后过期。</p>
<p>字符串中可以在添加键值的同时设置过期时间，并可以使用 <code>persist</code> 命令移除过期时间。同时我们也知道了过期键在 RDB 写入和 AOF 重写时都不会被记录。</p>
<p>过期键在主从模式下，从库对过期键的处理要完全依靠主库，主库删除过期键之后会发送 <code>del</code> 命令给所有的从库。</p>
<p>Redis 是通过设置过期字典的形式来判断过期键的，Redis 采用的是惰性删除和定期删除的形式删除过期键的，Redis 的定期删除策略并不会遍历删除每个过期键，而是采用<strong>随机抽取</strong>的方式删除过期键，同时为了保证过期扫描不影响 Redis 主业务，Redis 的定期删除策略中还提供了最大执行时间，以保证 Redis 正常并高效</p>
<blockquote>
<p>巨人的肩膀：</p>
<p><a target="_blank" rel="noopener" href="http://learn.lianglianglee.com/%E4%B8%93%E6%A0%8F/Redis%20%E6%A0%B8%E5%BF%83%E5%8E%9F%E7%90%86%E4%B8%8E%E5%AE%9E%E6%88%98/18%20Redis%20%E8%BF%87%E6%9C%9F%E7%AD%96%E7%95%A5%E4%B8%8E%E6%BA%90%E7%A0%81%E5%88%86%E6%9E%90.md">http://learn.lianglianglee.com/%E4%B8%93%E6%A0%8F/Redis%20%E6%A0%B8%E5%BF%83%E5%8E%9F%E7%90%86%E4%B8%8E%E5%AE%9E%E6%88%98/18%20Redis%20%E8%BF%87%E6%9C%9F%E7%AD%96%E7%95%A5%E4%B8%8E%E6%BA%90%E7%A0%81%E5%88%86%E6%9E%90.md</a></p>
</blockquote>

                
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